1. Field of the Invention
The present invention relates to a radiograph detector used for forming a radiograph, and a method for manufacturing this radiograph detector.
2. Description of the Related Art
A radiograph such as an X-ray image has been widely used for medical condition diagnosis given in medical sites. There has emerged in recent years a digital radiograph detecting device, such as a computed radiography (CR), and a flat panel radiograph detector (flat panel detector: FPD), as typical examples of the digital radiograph detecting device. In the field of digital technologies associated with X-ray images, various studies have been currently conducted particularly for the FPD. The FPD is equipped with a scintillator panel which converts X-rays into visible light. This scintillator panel includes a fluorescent (scintillator) layer which contains an X-ray fluorescent substance characterized by a function of converting applied X-rays into visible light and emitting the light. The FPD converts a visible light image generated from the scintillator panel into an electric signal using a thin film transistor (TFT).
Gadolinium oxysulfide (GOS) has been attracting attention as a material capable of constituting the fluorescent substance included in the FPD, based on the characteristics of gadolinium oxysulfide (GOS), i.e., no harmful heavy metal such as cadmium contained, and high machinability together with excellent optical characteristics such as high sensitivity and short afterglow. Accordingly, various research and development have been also conducted for the FPD containing a gadolinium oxysulfide (GOS) fluorescent substance as a fluorescent substance to be incorporated in the FPD.
The FPD which contains the gadolinium oxysulfide (GOS) fluorescent substance typically includes a fluorescent layer constituted by a gadolinium oxysulfide (GOS) fluorescent substance and disposed adjacent to a photoelectric conversion element panel via a bonding layer.
However, in case of the FPD configured to contain the fluorescent layer disposed adjacent to the photoelectric conversion element panel via the bonding layer, including the case of the FPD which contains a gadolinium oxysulfide (GOS) fluorescent substance as a fluorescent substance, delamination between the bonding layer and the fluorescent layer may occur due to insufficient bonding strength therebetween. In this case, durability of the FPD deteriorates. There is a type of FPD which contains a plurality of layers constituting a fluorescent layer. In this case, delamination between the plurality of layers may occur in addition to the foregoing delamination between the bonding layer and the fluorescent layer. For overcoming this problem of bonding strength, various ideas have been developed and proposed.
For example, JP 2012-189487 A discloses a radiographic imaging apparatus which includes a sensor panel, a scintillator panel, and a reinforcing plate in this order. According to this radiographic imaging apparatus, a second bonding layer disposed between the scintillator panel and the reinforcing plate is smaller in size than a first bonding layer disposed between the scintillator panel and the sensor panel. According to the description of the radiographic imaging apparatus disclosed in JP 2012-189487 A, stress produced by a difference between thermal expansion coefficients of a support substrate constituting the scintillator panel, and a sensor substrate constituting the sensor panel does not concentrate on an edge of the first bonding layer. Accordingly, delamination at the edge of the scintillator from the sensor panel caused by the stress is avoidable.
On the other hand, as a technology paying attention to an adhesive layer, JP 2012-202831 A discloses a radiograph detecting device which includes a radiographic conversion panel, an adhesive layer, and a sensor panel. The radiographic conversion panel contains a fluorescent substance, and a protection film for covering a fluorescence emitting surface of the fluorescent substance. According to the proposal described in this reference, the protection film is subjected to plasma treatment, while the adhesive layer is constituted by a hot-melt adhesive, for the purpose of improvement of both the image quality and durability of the radiograph detecting device. JP 2012-202831 A also describes that reduction of the thickness of the adhesive layer by a certain length can prevent deterioration of MTF (Modulation Transfer Function). According to JP 2012-202831 A, however, a layer constituted by column crystal such as cesium iodide is used as a fluorescent layer constituting the radiograph detecting device. Accordingly, no description is made in this reference concerning combination between the adhesive layer constituted by the hot-melt adhesive and a fluorescent layer constituted by fluorescent particles applied together with binder resin.
Japanese Patent No. 4307127 discloses a radiographic imaging apparatus which includes a sensor substrate containing photoelectric conversion image sensors, a fluorescent layer, and an adhesive layer disposed between the sensor substrate and the fluorescent layer, as a proposal of a technology for improving adhesion between the fluorescent layer and the adhesive layer and obtaining stable performance for a long period. According to this radiographic imaging apparatus, a binder polymer of the fluorescent layer and an adhesive layer forming polymer constituting the adhesive layer are cross-linked by a cross-linking agent contained in the adhesive layer. Japanese Patent No. 4307127 further describes the following points.
(i) The adhesive layer is chiefly constituted by an acrylic adhesive or the like, and obtained by applying application liquid which contains a mixture of a monomer mix and a cross-linking agent to a releasing film, and hardening the liquid for several hours. This adhesive layer in the form of an adhesive sheet provided with the releasing film is bonded to the fluorescent layer.
(ii) The adhesive layer thus formed is bonded to the fluorescent layer within 200 hours from formation of the adhesive layer to allow reaction between the cross-linking agent of the adhesive and binder resin of the fluorescent layer prior to dissipation of the function of the cross-linking agent. This method increases adhesion between the adhesive layer and the fluorescent layer.
(iii) After bonding between the fluorescent layer and the adhesive layer, the cross-linking agent contained in the adhesive layer is deactivated by exposure for 100 hours or longer at a temperature ranging from 20° C. to 30° C. Thereafter, the releasing film is released, and the fluorescent layer is bonded to the substrate surface of the photoelectric conversion image sensor via the adhesive layer to produce the radiographic imaging apparatus.
According to Japanese Patent No. 4307127, however, the bonding temperature at the time of bonding between the adhesive layer and the fluorescent layer lies approximately in the range from 50° C. to 80° C. Accordingly, this reference is not considered to present a technical idea of melting the chief component of the adhesive layer, and infiltrating the melted chief component into the fluorescent layer.